At present, a wide variety of IMDs are commercially released or proposed for clinical implantation in the human body. Certain IMDs are manufactured as discrete units that may be selected by an implanting physician for a particular clinical use, being coupled, at implantation, with one or more medical electrical leads. Such an IMD may be an implantable pulse generator (IPG) or a physiologic monitor and particular examples include but are not limited to cardiac pacemakers, cardioverter/defibrillators, cochlear implants, muscle and nerve stimulators, and deep brain stimulators.
IMD's typically include signal processing and/or pulse generating circuitry powered by a battery and enclosed within a hermetically sealed enclosure or housing, sometimes referred to as a “can”; hermetically sealed housings are typically formed of a conductive biocompatible metal, commonly titanium, that is corrosion resistant when exposed to body fluids during chronic implant. A connector header attached to a header mounting surface of the can enables coupling of the IMD with one or more leads, whereby electrical connection is made between lead electrodes and the circuitry enclosed within the housing. The connector header includes a connector bore adapted to receive a lead connector terminal and formed in a body fabricated of a relatively hard, dielectric, non-conductive polymer, which encases and isolates header connector elements that are mounted within the connector bore. Each header connector element is connected by means of an insulated feed-through passing between the header and the interior of the can to the circuitry therein. Typically the connector header body is transparent so that an implanting physician can observe, upon inserting a lead connector terminal into the header connector bore, the seating of the lead connector terminal within the bore in order to verify electrical connection between connector elements of the lead connector terminal and corresponding header connector elements.
Typically, electrical medical leads support at least one stimulation and/or sensing electrode and certain IMD's employ all or part of an exterior conductive surface of the can as an electrode functioning in conjunction with one or more of the lead electrodes for delivering stimulation energy and/or sensing electrical body signals and/or for sensing impedance changes in tissue. For the delivery of cardiac pacing pulses, the can may act as an anode or indifferent electrode in conjunction with a lead cathode, and, for the delivery of monophasic or biphasic cardioversion/defibrillation shocks, the can may act as a high voltage electrode in conjunction with at least one other lead electrode of an opposite polarity. In the latter case, it is desirable that the can electrode surface area be maximized and that its electrical characteristics remain stable over the battery lifetime. It has been observed that the surface characteristics of titanium change over time when titanium is exposed to body fluids and when stimulation energy is delivered through it, for example by oxidation. Normally, delivery of pacing pulses over the lifetime of the battery does not alter the titanium surface characteristics enough to materially affect pacing and sensing functions or shorten battery life; however, it has been found that delivery of cardioversion/defibrillation shocks through the can electrode accelerates surface oxidation and passivation and may negatively affect sensing of both electrical signals and impedance and may reduce the efficiency of energy delivery, particularly cardioversion/defibrillation shocks, enough to shorten battery life. Impaired sensing and reduced efficiency of delivery of cardioversion/defibrillation shocks may also occur as the surface area of the can electrode is diminished. Over the years, the mass and volume of IMD's have been progressively reduced even as their function and longevity have increased. The goals of reducing IMD volume along with simplifying assembly have led to proposals to minimize the size of the connector header or to enclose it within the hermetically sealed can. A connector header enclosed within the can may make verification of a fully seated lead connector terminal within the connector bore difficult. Thus, a need remains for a means to augment an electrode surface area of an IMD without impairing the visual verification of lead connections within an IMD connector bore(s).